Inspection processes are used at various steps during semiconductor manufacturing to detect defects on wafers to increase yield. However, as the dimensions of semiconductor devices decrease, inspection becomes more important to the successful manufacture of semiconductor devices because smaller defects can cause the devices to fail. Semiconductor manufacturers seek improved sensitivity to particles, anomalies, and other defect types, while maintaining overall inspection speed (in wafers per hour) in wafer inspection systems.
Each successive node of semiconductor manufacturing requires detection of smaller defects and particles on the wafer. Therefore, higher power and shorter wavelength UV (ultraviolet) lasers for wafer inspection are needed. Because the defect or particle size is reduced, the fraction of the light reflected or scattered by that defect or particle is also typically reduced. As a result, an improved signal-to-noise ratio may be needed to detect smaller defects and particles. If a brighter light source is used to illuminate the defect or particle, then more photons will be scattered or reflected and the signal-to-noise ratio can be improved if other noise sources are controlled. Using shorter wavelengths can further improve the sensitivity to smaller defects because the fraction of light scattered by a particle smaller than the wavelength of light increases as the wavelength decreases.
Some inspection tools for wafers and reticle inspection used in the semiconductor industry rely on deep-ultraviolet (DUV) radiation. Some of the most compact, efficient, and cost effective sources of laser radiation in the UV and DUV spectral regions are based on wavelength conversion of solid-state laser radiation in nonlinear optical crystals. When exposed to high-power DUV-radiation, optical components, including nonlinear optical crystals, are prone to optically induced damage, which limits the maximum power density present on or in each individual component. This power density limitation forces the optics designer to make trade-offs between achievable DUV-power, spatial beam quality, component lifetime, and form factor of the wavelength converter device. Optimizing the beam size in the nonlinear crystal may be needed to take advantage of and trade-off between harmonic (DUV) power, spatial beam quality, and nonlinear crystal lifetime. Meanwhile, a low power density on the optics may be needed to achieve the desired component lifetime.
If one or multiple of the wavelengths involved in the nonlinear wavelength conversion process are in the DUV region, the DUV-radiation is prone to cause optically induced damage not only in the nonlinear crystal, but also to other optical components in the beam shaping optics. Limiting the power density both in the crystal and on/in the beam shaping optics can become important in this case. The acceptable power density on the beam shaping optics can be considerably lower than in the nonlinear crystal itself. This requirement is in part due to material properties (e.g., in the case of fused silica) and in part due to the fact that the spot shifting schemes commonly used for nonlinear crystals cannot be applied to many optics components (e.g., spherical lenses) without introducing misalignment and aberrations into the beam path. This makes it necessary to position the beam shaping optics at a distance from the nonlinear crystal where the beam has diverged enough to reduce the optical power density to an acceptable level. Increasing the focus size in the nonlinear crystal decreases the beam divergence. The required distance from the nonlinear crystal to the beam shaping optics increases accordingly, so that the wavelength conversion module may become larger than desired.
The nonlinear crystal can be periodically shifted perpendicular to the beam, which uses multiple crystal locations. If one area of the nonlinear crystal is damaged, then the nonlinear crystal is moved relative to the beam so that the beam is projected onto a different, undamaged area. While this may prolong the period before the nonlinear crystal must be replaced, this fails to address the cause of any damage to the nonlinear crystal.
Therefore, what is needed is an improved nonlinear optical wavelength converter.